A navigation system performs travel guidance for enabling a user to easily and quickly reach the selected destination. For example, a vehicle navigation system guides a user (driver) of a vehicle to a destination through a calculated route. Such a navigation system detects a current position of the user's vehicle via a GPS (Global Positioning System) and calculates an optimum route to a destination specified by the user based on map data retrieved from a data storage medium, for example, a DVD (digital versatile disc), a semiconductor memory drive, or a hard disc.
Typically, the navigation system displays a map image on a monitor screen while superimposing thereon a mark representing the current location of the user' vehicle. At each intersection on the calculated route, the navigation system notifies the user which direction to turn. During such route guidance, in addition to GPS, the navigation system also performs dead reckoning and map matching technology with use of distance data from sensors attached to the vehicle, a gyroscope and accelerometers to improve reliability of the route guidance system.
A conventional navigation system uses a path finding algorithm to calculate a route between a start point (current position) to a destination. The path finding algorithm of a navigation system considers an overall cost, for example, a distance and a time to reach the destination. When there is a suitable freeway that may be used to reach the destination, such a freeway may be preferentially selected in a calculated route because of its low cost. Namely, freeways have a higher speed limit and do not have traffic lights or stop signs that slow down the vehicle so that the vehicle can reach the destination with a short time, i.e., with a low cost.
Typically, the cost is associated with a distance, road type, speed limit, left/right turn maneuver, entry/exit of a freeway, etc., and is used to calculate a desired route to the destination. When an entry to a freeway exists near a start point, the path finding algorithm often produces a calculated route that promptly enters the freeway entry since the vehicle can reach the destination in a shorter time. However, such a route may require the driver to encounter a freeway junction and to make a complex maneuver soon after the entry to the freeway. A traffic maneuver at a freeway junction can be stressful even for an experienced driver, especially when a maneuver is required soon after entering the freeway. Thus, it is desirable that the navigation system offers a route that is easy for a driver to follow.